The invention relates to a hydraulic cylinder for a hydraulic drawing cushion of a deep draw press.
During the drawing process, the drawing press clamps a metal sheet between an upper tool part and a lower tool part. By a relative movement between the upper tool part and the lower tool part, the metal sheet is pulled over a lower die and pressed between two die parts. The metal sheet retaining force required for the drawing is provided by the drawing cushion.
A drawing cushion for a drawing press is described, for example, in EP 0069 201 A2. The drawing cushion includes a cylinder which comprises three operating chambers. Two of the operating chambers operate pneumatically while the third operating chamber is in the form of a hydraulic operating chamber. The hydraulic operating chamber provides for the blocking of the drawing cushion in the lower end position and the controlling of the upward movement of the piston rod. The two pneumatic operating chambers are separated from each other by a differential piston. During the upward movement of the piston rod the two pneumatic operating chambers are placed in communications with each other, so that surface area difference of the differential piston becomes effective and moves the piston upwardly for the ejection of the formed metal part. This movement can be controlled by the oppositely acting hydraulic pressure in the hydraulic operating chamber.
Based hereon, it is the object of the invention to provide a compact hydraulic cylinder for a drawing cushion by which the position of the piston rod as well as the force provided by the piston rod can be controlled.